Coatings are applied to paper and fillers are utilized in paints or paper to improve various qualities including printing quality and optical properties such as brightness, opacity and gloss. Sodium aluminosilicates are well known as paper pigments. An important group of sodium aluminosilicates are the composite products produced by the hydrothermal reaction between kaolin clay and alkali metal silicates as described in U.S. Pat. No. 4,812,299. These composite products are described as altered kaolin clay platelets which have an integral rim or protuberance of essentially amorphous alkali metal silicate-kaolin clay reaction product. The composite compositions are structured materials in which the degree of structure is controlled depending on reaction conditions.
An improved structured sodium aluminosilicate composite paper pigment is described in PCT Publication WO92/03387 and U.S. Pat. No. 5,186,746. The structured sodium aluminosilicates described in these publications are produced by the reaction of sodium silicates and kaolin clay under hydrothermal conditions. The composite products are characterized by having low oil absorption values and high total pore volumes. These products offer high performance properties despite their low oil absorption characteristics.
The present invention provides an improved group of structured pigments which have optimum pore structures and improved slurry rheology at higher solids contents than the prior art.